1. Field of the Invention
The present invention relates generally to fluorescent lamp electronic ballasts, and more specifically to a cost effective structure for controlling fluorescent lamp current from an electronic ballast.
2. Description of the Prior Art
Incandescent lamps are relatively inexpensive compared to fluorescent lamp systems, but fluorescent lamps are more cost effective to operate because they can produce the same amount of light with much less electrical energy. A significant part of the initial cost of a fluorescent lamp system is the ballast. Fluorescent tubes express a negative resistance characteristic that must be controlled in order to maintain consistent light output and long life. Ballasts are also used to start the gaseous discharge tubes and to generate the required voltages, which are substantially higher than ordinary 110 VAC.
Integrated circuit technology has provided for lower cost ballasts and therefore the cost of a fluorescent lamp system. Ballasts are produced and used in very high quantities and a savings of one or two dollars per ballast can account for millions of dollars of savings and can tip the balance between a choice of incandescent or fluorescent lights. Generally, as the functions integrated onto a chip increase, the cost of a ballast decreases.
Electronic ballasts generally employ an oscillator that sets the fundamental frequency of output switching transistors that drive a transformer output network. The output network will deliver power to the fluorescent tubes that varies with the frequency of the oscillator and the value of resistors, capacitors, and inductors in the ballast and output network. Production variances in component values can lead to inconsistent light output from unit to unit if not otherwise controlled. A common, albeit relatively expensive, method of controlling lamp performance is to measure or sample the output current flowing through the lamps and to feed that sample current back to a current controlling oscillator. The oscillator may be voltage controlled since the frequency output will directly control output current.
Output current feedback control is expensive because it requires components that cannot readily be integrated on a chip and typically requires a winding on the output transformer or sense transformer to sense output current. Precision oscillator frequency control in the prior art has also been costly because such precision required the use of sophisticated oscillators and crystals.
Thus, there is a need for a structure providing precision oscillator frequency control that can be readily integrated into a semiconductor chip and that limits the effects of production variances that may occur in component values.